Pandemic (Scott Sigler)

The alien intelligence that unleashed two horrific assaults on humanity has been destroyed. But before it was brought down in flames, it launched one last payload - a tiny soda-can-sized canister filled with germs engineered to wreak new forms of havoc on the human race. That harmless-looking canister has languished under thousands of feet of water for years, undisturbed and impotent . . . until now.
Days after the new disease is unleashed, a quarter of the human race is infected. Entire countries have fallen. And our planet's fate now rests on a small group of unlikely heroes, racing to find a cure before the enemies surrounding them can close in.